It is known in the art relating to traction motor drives of the type used in locomotives to employ a motor having a helical pinion at each end for driving corresponding gears fixed to an axle. The pinions and gears are secured to their respective shaft and axle only by an interference fit. Such a drive is shown in FIG. 1 and includes a motor 10 having a housing 12, stator windings 14 within the housing and a rotor 16 having an output shaft 18 journaled in cylindrical roller bearings 20. Since the cylindrical bearings 20 offer no axial thrust, the rotor is able to move axially throughout a limited range of end play, typically about 0.05 to 0.10 inch. The left end of the shaft 18 carries a pinion 22L and the right end of the shaft 18 carries a pinion 22R. The pinions are both of the helical tooth form type and have opposite helix angles.
The motor 10 is mounted via flanges 24 on the housing 12 to corresponding flanges 26 on an axle housing 28. An axle 30 is journaled in the housing 28 by tapered roller bearings, not shown, which hold the axle 30 against any significant end play. The outboard ends of the axle support wheels 32. Just inboard of the wheels the axle has twin helical gears 34L and 34R which mesh with the pinions 22L and 22R, respectively, and thus have opposite hand and matching helical angles. A square cross section key 36 fits in aligned keyways formed in the axle housing 26 and the motor housing 12 to fix the relative axial positions of the housings, and bolts 38 through the flanges 24 and 26 hold the housings together.
The opposite tooth angles of the right and left pinions and gears have the effect during driving of developing an axial thrust on each pinion which opposes the thrust of the other pinion, and the rotor then tends to shift axially in its housing 12 if the thrust forces are not equal. The rotor seeks a position where the thrust forces equalize, if the end play of the rotor is sufficient to allow movement to such a position, and therefore the same driving torque is transmitted to each gear. Thus, for a given assembly, the axle position determines the rotor position. Due to unequal backlash in the two sets of gears and pinions, the axial rotor position may be slightly different for forward and reverse drive, and thus the rotor will shift axially upon change of drive direction. If the limit of axial travel of the rotor is reached before the thrust forces equalize, the rotor will rub inside the housing.
To accommodate the necessary axial shifting of the rotor as well as dimensional changes due to thermal expansion, the gears and pinions should be indexed so that for the condition of equalized backlash on both ends the rotor should be substantially centered in its range of end play. This centered condition is depicted geometrically in FIG. 2. Pinion teeth 34R and 34L are shown between gear teeth 22R and 22L, respectively. The angle of each tooth face is projected by dashed lines which meet at apex A1 for the tooth faces of one surface of 22L and 22R, and apex A2 for adjacent faces of pinions 34L and 34R. In the example shown, the tooth spacing of the gear teeth 22R is greater than the tooth spacing of gear 22L. When the backlash is equalized or centralized on each end of the assembly, there is a backlash B between each pinion 34R and the adjacent gear 22R and a backlash C between the pinions 34L and gears 22L. The amount of backlash may be on the order of 0.015 or 0.030 inch and for the general case the backlash B is different from the backlash C. Then the apices of the adjacent teeth are not aligned when in the equalized backlash condition but when driving occurs, the torque causes end thrust which moves the rotor endwise sufficiently to align the apices for engaging tooth faces, e.g. A1 and A2. The range of end play P is shown for the case of misalignment such that the rotor position as shown by the apex A2 is considerably spaced by an offset D from the center of the range of end play P. The misalignment severely limits the axial travel of the rotor to the left and may prevent the alignment of apex A2 with apex A1. Thus it is important to assemble or adjust the gear sets so that the rotor is centered within the stator when the backlashes are equalized to afford permissible travel in either direction to effect forward or reverse drive. This will allow the pinion thrusts to become equalized by axial shifting of the rotor when torque is applied.
In the past it has been the practice to manufacture one of the gears 34L or 34R in two pieces comprising an outer toothed gear ring and an inner hub, secured by 18 bolts. The bolts were threaded into one of the pieces and the other piece had an arcuate slot for each bolt to allow angular adjustment. The motor housing 12 would be bolted to the axle housing with the key 36 maintaining the housings at a fixed relative position. The rotor would be locked in the center of its end play and the 18 bolts on the two-piece wheel would be loosened, the gear ring and hub would be angularly adjusted to equalize the gear backlash, and the bolts would be tightened. This laborious process was necessary for each new assembly, and if a motor were to be changed to another axle, the process would have to be repeated to index the gears again.